1. Field of the Invention
This invention relates to an electrochemical sensor device for detecting the presence of chemical analytes which are in either a liquid or vapor phase. More specifically, the invention relates to a reversible electrochemical sensor comprised of conductive polymer composite materials and the method of making conductive polymer composite materials for reversible electrochemical sensors.
2. Description of the Prior Art
Chemical sensing, and in particular chemical solvent sensing, has become very important for environmental and loss management concerns. The ability to detect a leak or the presence of a chemical and to identify the chemical in an inexpensive manner is of great interest. Commercial and industrial establishments concerned about gaseous emissions or chemical spills, as well as owners or operators of underground installations or utilities such as fiber optical cables, which can be damaged in the presence of chemical solvents, have a need for reliable and inexpensive chemical sensors.
Conductive polymers and conductive polymer composites have been used for chemical sensing applications because of their ability to be tailored to the chemical(s) to be sensed by a judicious choice of polymer, polymer quantities and constituents. Electrochemical sensors employing conductive polymers and conductive polymer composites often exhibit a change in conductivity in the presence of a target chemical(s). The mechanism effecting the conductivity change is often a swelling of the polymer when it absorbs the chemical or chemical vapors. This swelling alters the volume concentration of the polymer resulting in an increase in the distance between one conductive network branch to the next; therefore changing the conductivity of the polymer.
One such device is illustrated and described in U.S. Pat. No. 5,417,100 (Miller et al.) which discloses a reversible sensor for detecting solvent vapors. The sensor of the '100 patent consists of a dielectric substrate; a pair of interdigitated, electrically conductive electrodes disposed on the surface of the substrate; and a composite coating covering the interdigitated electrodes and comprising a conductive polymer and a dielectric polymer with an affinity for the solvent vapors to be detected. The sensor of the '100 patent relies on physical absorption of the vapor being detected. The absorbed vapor causes the conductive polymer composite to swell, increasing the distance between the conductive polymer chains, and therefore exhibiting a loss in conductivity, or increase of volume resistivity in the composite.
U.S. Pat. No. 5,698,089 (Lewis et al.) discloses a chemical sensor for detecting analytes in fluids. This sensor consists of a pair of conductive elements (electrical leads) coupled to and separated by a chemically sensitive resistor which provides an electrical path between the conductive elements. The resistor comprises a plurality of alternating nonconductive regions of a nonconductive organic polymer and conductive regions comprised of a conductive material. The electrical path length and resistance between the conductive regions changes with the absorption of analytes. The '089 patent also teaches of sensor arrays incorporating combinations of sensors having varied polymer and conductive polymer constituents so as to have sensitivity to a variety of analytes.
The patent to Soane (U.S. Pat. No. 5,672,297) teaches of a gel-matrix whose electrical and/or thermal conductivity undergoes a significant change in response to minor variations in one of several externally controlled thermodynamic parameters such as temperature, pH, ionic strength and solvent composition. The gel-matrix is comprised of three primary components: conductive particles, swellable and deswellable crosslinked particles, and a solvent system. In the de-swollen state, the conductive particles are normally discrete. When the gel-matrix is swollen (in response to a variation in temperature, pH, ionic strength or solvent composition), the interstitial volume between cross-linked gel particles diminishes, forcing conductive particles to come into intimate contact with one another, thus creating a more conductive network.
The heretofore discussed '100 and '089 patents prefer the use of intrinsically conductive polymers, such as polyaniline and polypyrrole, for the conductive polymer regions; while the '297 patent shows a preference for a variety of metallic and other conductive particles, including carbon black powder, for the conductive filler. The application of composite conductive polymers as electrochemical sensors using carbon black as the conductive filler has also been reported on. See for example, Lundberg and Sundqvist (1986) J.Appl.Phys. 60:1074-1079, the contents of which are incorporated by reference, which reports on the resistivity of a polyethylene matrix with a carbon black filler and a poly(tetrafluoroethylene) matrix with a carbon black filler as a function of exposure to various solvents. It was found that the resistivity of these composites increased when exposed to certain solvent vapors or were immersed in certain solvents. The report also suggests combining two or more composites with sensitivities to different analytes in a single detector, thus forming a versatile electronic sensor array.
The ability of polymers to act as electrical insulators is the basis for their widespread use in the electrical and electronic fields. However, material designers have sought to combine the fabrication versatility of polymers with many of the electrical properties of metals. A few select polymers, such as polyacetylene, polyaniline, polypyrrole and others, can be induced to exhibit intrinsic conductivity through doping, although these systems tend to be cost prohibitive and difficult to fabricate into articles. An extrinsic approach of imparting conductivity to a polymer is through the creation of conductive polymer composite ("CPC") materials. CPC materials require a random distribution of a conductive filler to be dispersed throughout an insulating polymer which results in an infinite network capable of supporting electron flow. Prior art CPC materials have employed metals, intrinsically conductive polymers, or, most often, carbon black as the conductive filler.
A crucial aspect in the production of CPC materials is the quantity of conductive filler content. If the quantity of conductive filler is too high, the processing becomes difficult, the mechanical properties of the composite are poor, and the final cost is high. Therefore, the quantity of conductive filler should be as low as possible while still allowing the composite to fulfill its electrical requirements.
Percolation theory has been successfully used to model the general conductivity characteristics of CPC materials by predicting the convergence of conducting particles to distances at which the transfer of charge carriers between them becomes probable. The percolation threshold ("p.sub.c "), defined as the lowest concentration of conducting particles at which continuous conducting chains are formed, can be determined from the experimentally determined dependence of conductivity of the CPC material on the filler concentration. For a general discussion on percolation theory, see Kirkpatrick (1975) Review of Modern Physics 45:574-588, the contents of which are herein incorporated by reference. Much work has been done on determining the parameters influencing the p.sub.c with regard to the conductive filler material. See for example Lux (1993) J. Materials Sci. 28:285-301; Narkis and Vaxman (1984) J.Appl.Poly.Sci. 29:1639-1652; and Sherman and Middleman, et al. (1983) Poly.Egr. & Sci. 23:36-46; the contents of each of which are herein incorporated by reference.
A typical method of optimizing the conductive filler level to conductivity performance ratio of CPC materials is to reduce the content of the conductive filler to a value just above p.sub.c. More recently, this work has been advanced by developing approaches which exploit aspects of percolation to significantly reduce p.sub.c while maintaining high levels of macroscopic conductivity. These more recent approaches realize the reduction in p.sub.c by promoting phase inhomogeneities in the total material. For example, in a binary mixture of a semicrystalline polymer and a conductive filler, the filler particles are rejected from the crystalline regions into the amorphous regions upon recrystallization, which accordingly decreases the p.sub.c. Similarly, using a polymer blend with immiscible polymers which results in dual phases as the matrix in CPC materials is another alternative to promoting phase inhomogeneities and lowering the p.sub.c. The heterogeneous distribution of the conductive filler within the polymers is a crucial parameter in this latter example. In one alternative of this approach, either one of the two polymer phases is continuous and conductive filler particles must be localized in the continuous phase. In a second alternative, the two phases are co-continuous and the filler is preferably in the minor phase or more preferably at the interface. These alternatives of dual continuity or "double percolation" have been reported in the scientific literature, see for example Levon and Margolina, et al. (1993) Macromolecules 26:4061-4063, the contents of which is herein incorporated by reference.
Applications of the heretofore described alternatives for reduction of conductive filler content in CPC materials have been reported for polyethylene/polystyrene immiscible blends and for polypropylene/polyamide immiscible blends, both employing carbon black as the conductive filler. See for example, Gubbels and Blacher, et al. (1995) Macromolecules 28:1559-1566; and Tchoudakov and Breuer, et al. (1996) Poly.Egr. & Sci. 36:1336-1346, the contents of both of which are herein incorporated by reference.
Accordingly, while the prior art teaches dual continuity or "double percolation" as a method for reducing the p.sub.c, the prior art concerned with double percolating systems has not taught how to fully reduce the conductive filler content in CPC materials through a judicious choice of materials and various processing approaches to improve the conductive network. Further, the prior art concerned with double percolating systems has not taught or suggested how to crosslink these CPC materials. Moreover, the prior art has not recognized the potential of utilizing optimized double percolating systems for electrochemical sensors or for combining multiple sensors into one CPC by a percolation-within-percolation approach, hereinafter described.
Therefore, it is an object of the present invention to provide an electrochemical sensor whose electrical conductivity undergoes a reversible change in response to variations in solvent composition.
It is a further object of the present invention to provide a class of reversible electrochemical sensors whose sensitivity and selectivity can be tailored by a judicious choice of materials.
It is another object of the present invention to provide a reversible electrochemical sensor which incorporates multiple sensors into one CPC through a percolation-within-percolation approach to fabrication.
It is a further object of the present invention to provide a reversible electrochemical sensor, comprised of a crosslinked CPC fabricated by the percolation-within-percolation approach, whereby the sensor retains its reversibility even in environments where the sensor is subject to aggressive solvents for long periods.
It is a further object of the present invention to provide the foregoing objects with an inexpensive CPC.
It is also an object of the present invention to provide a method of making the electrochemical sensors of the foregoing objects.
The heretofore stated objects are achieved in part by basing the reversible electrochemical sensor of the present invention on a CPC material of an immiscible polymer blend having reduced conductive filler content by decreasing the p, required to generate a continuous conductive network in the CPC and by judicious selection of materials which have an affinity for swelling in the presence of a target analyte.